The inkjet printing method is a method in which ultra fine liquid droplets of an ink composition are adhered to a printing substrate, thereby recording images and/or text, and is characterized by not requiring the use of a plate in the printing process. The electrophotographic method is another well known printing method that does not use a plate, but in terms of apparatus cost, running costs and printing speed and the like, the inkjet printing method is considered to be superior. With the increase in demand for on-demand printing seen in recent years, the demand for inkjet printers continues to grow.
The ink compositions used in this type of inkjet printing require far more stringent control of the viscosity than the ink compositions used in offset printing or gravure printing. This is because a change in the viscosity of the ink composition causes a change in the liquid droplet volume during discharge, resulting in a change in the image quality of the printed item. Moreover, in recent years, inkjet printing systems have become widely used throughout the world, and ink compositions that suffer minimal changes in quality, and particularly viscosity, during long-term transport or storage, namely ink compositions that exhibit excellent stability over time, are now being demanded.
On the other hand, the ink compositions used in inkjet printing encompass many types of compositions, including solvent-based compositions, water-based compositions, and active energy beam-curable compositions. Among these, there has recently been a increase in the demand for active energy beam-curable inks, because they can be cured rapidly by irradiation with an active energy beam and therefore exhibit excellent printing speed, can also be applied to non-absorbent substrates such as plastic and board, and can minimize volatilized solvent volume and are therefore environmentally friendly. Particularly in the case of inkjet printing for commercial applications and industrial applications, the consideration of other additional factors, including the strength and durability of the printed product, the drying energy of the ink, and resistance to adherence of the ink components to the print head caused by ink drying, means that the use of active energy beam-curable inks is becoming increasingly widespread.
However, it is known that compared with inkjet ink compositions of other types, controlling the viscosity and the stability over time is more difficult for active energy beam-curable inkjet ink compositions. This is because the main component of an active energy beam-curable inkjet ink composition is a polymerizable monomer, and minute amounts of polymerization initiating components generated during transport or storage can cause the polymerization reaction of the polymerizable monomer to proceed.
In other words, in developing an active energy beam-curable inkjet ink composition, the important question is how to control the viscosity and stability over time, while maintaining the curability (productivity) and quality such as the adaptability to different substrates that represent the major advantages of such inks.
A multitude of previous investigations have already been undertaken with the aim of addressing the above problems. For example, Patent Document 1 is an example in which an organic solvent such as a petroleum-based solvent is added to the ink composition to improve the storage stability. However, in this example, because an organic solvent that does not participate in the polymerization reaction is added, the curability worsens. In addition, because adding an organic solvent eliminates one of the advantages of using an active energy beam-curable inkjet printing method, namely the ability to reduce the volume of volatilized solvent, this approach is undesirable. Further, Patent Document 2 is an example in which an N-vinyl lactam is used as the polymerizable monomer, and an antioxidant and a polymerization inhibitor are added in an attempt to improve the storage stability. However, the ink compositions disclosed in the specification have an extremely high content of a polymerizable monomer containing only one polymerizable reactive group (monofunctional monomer). As a result, the strength and durability of the printed item, which should be advantages of an active energy beam-curable inkjet printing method, tend to deteriorate, which is undesirable. Moreover, Patent Document 3 is an example in which, in order to improve the storage stability, a pH buffer is added to the ink composition in an attempt to improve the stability over time. However, the Good's buffer disclosed in this document is an ionic compound. As a result, depending on the storage conditions, there is a possibility that the buffer may form a different ionic compound with a material in the ink composition, thereby generating a precipitate which may appear in the inkjet head, which is also undesirable.
As described above, an ink composition that has low viscosity yet exhibits superior curability and substrate adhesion, and also has favorable storage stability has not yet been found.